This invention relates to a radio frequency coil, and more particularly to an adjustable radio frequency coil for use in a nuclear magnetic resonance imaging apparatus for medical diagnosis.
A radio frequency coil for nuclear magnetic resonance imaging is in effect a radio frequency antenna which is used to generate a radio frequency magnetic field for exciting the nuclei in a sample, as well as to receive the radio frequency signals which are emitted by the excited nuclei. FIG. 1 illustrates a conventional radio frequency coil of the type described in the Journal of Magnetic Resonance, Volume 36, pages 447-451 (1979). The illustrated coil comprises a pair of H-shaped members 1 which are made of plates of an electrically conducting material such as copper. The H-shaped members 1 are symmetrically disposed about a longitudinal axis so as to form a hollow, cylindrical tube. Each H-shaped member 1 has a longitudinally-extending web 1a, which is referred to as a vertical band, and two laterally-extending flanges 1b at opposite end of the web 1a, the flanges 1b being referred to as wings. Adjacent flanges 1b are electrically connected with one another by means of chip capacitors 2. Two electrically conducting guard rings 3a and 3b, at least one of which is grounded, are coaxially disposed within the tube at its opposite ends. The guard rings are separated from the inner surface of the H-shaped members 1 by a layer of an electrically insulating material such as Teflon. The guard rings shield a sample from the electric fields which are generated by the radio frequency voltage present between the wings and decrease dielectric losses within a sample. Two electrodes 4a and 4b are integrally formed on one of the H-shaped members 1 and on the bottom guard ring 3b, respectively. The electrode 4a which is formed on one of the H-shaped members 1 is connected to the hot electrode of a terminal 6 through a first impedance-matching variable capacitor 5a, while the other electrode 4b which is formed on the bottom guard ring 3b is directly connected to the ground electrode of the terminal 6. A second impedance-matching variable capacitor 5b is connected across the two electrodes of the terminal 6. The terminal 6 is connected via an unillustrated coaxial cable to an unillustrated radio frequency transceiver which is used to excite the coil and to receive the signals which the coil picks up from a sample.
The H-shaped members 1, the chip capacitors 2, and the guard rings 3a and 3b together form a three-dimensional circuit which is equivalent to a series resonant circuit, the resonant frequency of which can be adjusted by the variable capacitors 5a and 5b.
This conventional radio frequency coil has the drawback that no consideration is given to the balance of the currents which are fed to the electrodes 4a and 4b. Namely, in-phase currents are supplied to the terminal 6 and thus to the electrodes 4a and 4b via the coaxial cable, with the result that the operation of the coil is unstable, its value of Q is relatively low, and its sensitivity of reception is poor.